The recording density of an optical storage device including magneto-optical disks is dependent on the size of a light spot formed on the recording medium by a light beam which is used for recording and reproducing. Recently, a reproduction method has been proposed for reproducing recorded bits with sizes smaller than the size of a light beam.
Normally, the light beam for use in optical recording is converged to a diffraction limit by a converging lens; therefore, the light intensity distribution shows a Gaussian distribution, and thus the temperature distribution on the recording medium also exhibits a virtual Gaussian distribution. As a result, a portion on the recording medium having a temperature that is not less than a predetermined temperature become smaller in size than the size of the light spot. Consequently, the recording density can be greatly improved if only this portion can be utilized for reproduction.
Referring to FIG. 13, the following description will discuss a magneto-optical disk wherein a recorded bit with a size smaller than the size of a light spot can be reproduced.
The magneto-optical disk is mainly constituted by a readout layer 30 and a recording layer 31. The recording layer 31 has a great coercive force at room temperature. On the other hand, the readout layer 30 has a small coercive force at room temperature. When the temperature of a portion of the readout layer 30 to be reproduced is raised by irradiation of a light beam, the direction of the magnetization of the readout layer 30 becomes coincident with the direction of the magnetization of the recording layer 31 due to the effect of the recording layer 31. That is, the magnetization of the recording layer 31 is copied onto the readout layer 30 by an exchange coupling force between the readout layer 30 and the recording layer 31.
In the above arrangement, recording is executed by the ordinary optical-thermomagnetic recording method. When the recorded bits are to be reproduced, it is necessary to initialize the direction of magnetization of the readout layer 30 so as to make it coincident with the predetermined direction (upward in FIG. 13) by applying an auxiliary magnetic field from an auxiliary magnetic field generating device 32. Then, by projecting a reproduction-use light beam 33 thereonto, the temperature of the recording layer 31 is locally raised and the magnetized information is copied onto the readout layer 30. Thus, only the information located in the central portion which has received the reproduction-use light beam and undergone a temperature rise, is reproduced. Accordingly, recorded bits with sizes smaller than that of the light beam are permitted to be read out.
However, in the above-mentioned conventional arrangement, prior to a reproducing operation, an auxiliary magnetic field needs to be applied from the auxiliary magnetic field generating device 32. Further, during reproduction, a recorded bit that has been copied onto the readout layer 30 from the recording layer 31 remains as it is even after the temperature of the portion has cooled off. When the light beam is shifted so as to reproduce the next recorded bit, the recorded bit previously copied still exists within the light beam, and this residual bit tends to be reproduced. This causes noise and thereby presents a problem in improving the recording density.
Now, there are two recording methods for magneto-optical recording disks, that is, the light modulation method and the magnetic field modulation method. In the light modulation method, the size of recorded bits is dependent on the size of a light spot. On the other hand, in the magnetic field modulation method, although the width of recorded bits is dependent on the size of a light spot, the length of the recorded bits is not dependent on the light spot but on the recording frequency of the magnetic field; therefore, recorded bits each of which is shorter than the diameter of the light spot can be formed. However, the recorded bits thus formed have a circular arc shape; this results in a problem that the shape is not suitable for reproduction by the use of a round-shaped light beam.
The inventors of the present application, therefore, have invented a magneto-optical recording medium wherein a readout layer having the following characteristics is formed on a recording layer (see Japanese Laid-Open Patent Application 3-85503):
(1) Exhibiting the in-plane magnetization characteristic at room temperature (here, the in-plane magnetization characteristic is a magnetic characteristic whereby the axis of easy magnetization becomes in parallel with the recording layer.) PA1 (2) Having a Curie temperature that is substantially higher than that of the recording layer. PA1 (3) Having a magnetic characteristic that is altered from the in-plane magnetization characteristic to the perpendicular magnetization characteristic when the temperature of the readout layer is raised above a predetermined temperature through irradiation of a light beam, and thereby the magnetization direction of the readout layer becomes coincident with the magnetization direction of the recording layer. PA1 (1) Having a recording layer whereon information is magneto-optically recorded by the magnetic field modulation method. PA1 (2) Having a readout layer with a Curie temperature that is substantially higher than that of the recording layer, wherein an axis of easy magnetization is parallel to the recording layer at room temperature, and varies to be perpendicular to the recording layer within a predetermined temperature range between room temperature and the Curie temperature. PA1 (3) The readout layer being laminated on the recording layer, and located at the same side of a light source. PA1 (1) First irradiation means for irradiating the first light beam onto the readout layer to form the first light spot for raising the temperature of the readout layer. PA1 (2) Second irradiation means for irradiating the second light beam to form the second light spot for detecting the magneto-optical effect of the readout layer.
Here, it still remains as one of technical subjects to provide an optical head device for reproducing information from such a magneto-optical recording medium.